1. Field of the Invention
The present invention relates to a discharge tube for use as a light source and, more particularly, to a discharge tube serving as a light source like a xenon short arc lamp, a mercury-xenon lamp, or the like.
2. Related Background Art
Examples of such discharge tubes include those disclosed in Japanese Patent Application Laid-Open No. S60-131751 (which will be referred to hereinafter as Reference 1), Japanese Utility Model Application Laid-Open No. S61-90157 (which will be referred to hereinafter as Reference 2), Japanese Patent Application Laid-Open No. H01-213952 (which will be referred to hereinafter as Reference 3), Japanese Patent Application Laid-Open No. H08-273622 (which will be referred to hereinafter as Reference 4), Japanese Patent Application Laid-Open No. H09-92201 (which will be referred to hereinafter as Reference 5), and Japanese Patent Application Laid-Open No. H09-129179 (which will be referred to hereinafter as Reference 6).
Reference 1 discloses the discharge tube for use as a light source having a cathode in which a porous. metal substrate of tungsten or the like is impregnated with an electron-emissive material such as alkaline earth metals or the like. This Reference 1 describes that stable and good discharge is implemented by the use of the metal electrode impregnated with the electron-emissive material.
Reference 2 discloses the discharge tube in which only a sharp end of a sharply tipped, impregnated. cathode is coated with a metal to lower the work function, such as iridium or the like. This Reference 2 describes that the coating of the metal to lower the work function only on the sharp end can decrease deterioration of the electrode and stabilize electric discharge.
Reference 3 discloses the discharge tube having the cathode in which a coating of a refractory metal is formed over the entire surface without exposing the surface at a cusped tip of the cathode with a cusp. This Reference 3 describes that the thin-film coating of the refractory metal over the entire surface can stabilize the arc and decrease fluctuation of the arc.
Reference 4 discloses the cathode for discharge tube in which a metal electrode like a sharp, thin tungsten wire is buried along the axial direction in the central part of the impregnated cathode. This Reference 4 describes that the lifetime can be extended, because there exists no impregnant in the metal electrode in the center.
Reference 5 discloses the cathode for discharge tube in which a sharp electrode in the central part of the impregnated cathode is made of a porous metal material and in which the outer surface of an impregnated metal portion surrounding this center electrode is coated with a refractory metal. This Reference 5 describes that operating temperatures can be lowered because of the porous structure of the center electrode under supply of the electron-emissive material from the surrounding impregnated metal portion and that evaporation of the electron-emissive material can be suppressed by the coating on the outer surface.
Reference 6 discloses the cathode in which a mixture of a refractory metal and an electron-emissive material is stuffed into the inside of a cap-shaped metal case with a conical bored tip. This Reference 6 describes that the structure can decrease evaporation and wear of the cathode.
A variety of improvements and ideas have been presented heretofore concerning the cathodes in the discharge tubes as light sources as described above, but there is no cathode available yet to satisfy the various requirements for the discharge tubes as light sources, including easiness of production, stability of discharge, long lifetime, and so on, without any contradiction between them.
For example, in Reference 3 proposed as solution technology to the problems in References 1 and 2, the evaporation of the electron-emissive material is intended to be prevented by coating the entire surface of the tip part of the cathode with the refractory metal. This structure, however, makes it difficult to implement good emission of electrons from the cusped tip and, as detailed hereinafter in comparison with the present invention, it fails to restrain the evaporation of the electron-emissive material by the above coating after all.
In Reference 4, the sharp metal electrode of the tungsten wire is exposed at the tip of the cathode, but this makes it difficult to emit electrons at low temperatures from the cusped tip. Then Reference 5 suggests the cathode in which the center electrode is porous and is surrounded by the metal containing the electron-emissive material, but this makes it difficult to implement good electron emission from the cusped tip, as described hereinafter in comparison with the present invention.
On the other hand, Reference 6 describes that the evaporation of the electron-emissive material can be prevented by the covering of the metal cap, but it is not easy to produce the tip part of the cathode in this structure in the level of practical use. Particularly, the temperatures at the cusped tip become high around 1500xc2x0 C. during operation and it is difficult to press the porous metal into the metal cap so as to implement stable emission of electrons at such high temperatures over the long term.
An object of the present invention is, therefore, to provide a discharge tube for use as a light source that can realize the stability of discharge, long lifetime, easiness of production, etc. without any contradiction.
A discharge tube of the present invention comprises a vessel filled with a discharge gas, a cathode placed in this vessel and having a tip portion fixed to a lead rod, and an anode placed opposite to the tip portion of the cathode in the vessel; the cathode has a cusp pointed toward the anode, a metal substrate of an impregnated type in which a porous, refractory metal is impregnated with an electron-emissive material or a sintered type in which a refractory metal containing an electron-emissive material is sintered, and a coating of a refractory metal which covers a surface on the base end side of the cusp within.a surface of the metal substrate; an exposed portion in which the metal substrate is exposed without being covered by the coating is provided at a tip portion of the cusp of the cathode; or, the coating is made so thin or porous as to substantially expose the metal substrate at the tip portion of the cusp of the cathode.
According to the present invention, the tip portion of the cathode is made of the metal substrate containing the electron-emissive material or impregnated therewith, and the surface thereof on the base end side of the cusp is covered by the coating of the refractory metal; therefore, the evaporation of the electron-emissive material is prevented during operation. On the other hand, since at the tip portion of the cusp the metal substrate is exposed without being covered by the coating of the refractory metal or the coating is made so thin or porous as to substantially expose the metal substrate containing the electron-emissive material or impregnated therewith, electron emission is promoted by the electron-emissive material diffusing to the tip portion of the cusp. For this reason, electrons are emitted efficiently at relatively low temperatures, so as to stabilize the discharge, and the evaporation of the electron-emissive material is also prevented, so as to permit attainment of longer lifetime. Further, the tip portion of the cathode of the present invention can be realized in the simple structure wherein the coating of the refractory metal is formed on the surface of the metal substrate except for the tip portion of the cusp, and this permits provision of the discharge tube as a light source with high practical utility.
The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.